1. Field of the Invention
The present invention relates to a method for controlling the carrier of an amplitude-modulated transmitter. Such a method is known from Swiss patent No. 634 183.
2. Discussion of Background
In the known method, the useful Low-frequency signal present at the input of the transmitter and to be transmitted via the transmitter is converted, on the one hand, into a sequence of pulse-duration-modulated pulses which are used to drive a switching amplifier to conduct and not to conduct. On the other hand, a part of the incoming useful signal is branched off and rectified. The audible alternating currents are then filtered out in a filter chain. Following the filter chain, a slowly variable direct voltage remains which corresponds to the variation of the dynamic range of the useful signal. This slowly variable direct voltage is superimposed on the useful signal before it is converted into pulse-duration-modulated pulses.
During the subsequent pulse-duration-modulation process, functioning in accordance with the familiar saw tooth method, the superimposition of the slowly variable direct voltage leads to a dynamic-range-dependent displacement of the operating point which becomes noticeable a dynamic-range-dependent direct-voltage component at the output of the switching amplifier. During the subsequent modulation of the carrier oscillation, this direct-voltage component in turn influences the operating point on the modulation characteristic resulting in a direct dynamic-range-dependent control of the carrier.
The described method of carrier control according to the prior art is based on generating an analogous dynamic-range-dependent direct voltage by rectification and subsequent filtering of the useful signal. It is especially this filtering, that is to say the time-averaging over the variable amplitude of the useful signal which results in serious restrictions for the rapidity of response of the carrier control system with pulse-shaped changes of this amplitude of the useful signal (so called "tone bursts"). The carrier amplitude can follow such pulseshaped changes only within the modest framework predetermined by the large time constant of the filter chain.
This inertia of the familiar carrier control system leads to not negligible distortions in the transmission to the useful signal to the receiver, producing a clear loss in transmission quality.